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  • Users: BigJohn1
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  1. BigJohn1

    Does grounding a wye increase inrush?

    I had to ground the low-side wye on a 5MVA 13,200Δ:480Y transformer, afterwards we began tripping out on inrush when trying to energize with no secondary load. Nothing else about the installation changed except bonding the 480V wye to ground. I don't know what inrush was when the wye was...
  2. BigJohn1

    Partial Discharge in Molded MV Products

    I can't see any rhyme or reason for what will cause partial discharge in medium voltage cabling systems: I have noticed that a lot of molded splice and termination products have very significant internal gaps between the conductor and the molding as part of their design. These seem to work...
  3. BigJohn1

    Impending Generator Failure

    Have a 12MW utility-parallel, enclosed generator that is suddenly producing a very significant "hot electrical" smell. All instruments are reporting "normal" values for temperature, current, voltage, etc. No protection has operated. Did winding resistance and insulation resistance on both...
  4. BigJohn1

    Vacuum Breaker Prestrike Blowing Fuses?

    Very unusual problem on a 25MVA transformer 34.5kV Y : 13.8kV Δ. Has been in service for many years. Recent unexplained feeder fault blew a fuse. Now every single time the transformer is energized via the vacuum breaker it blows those same upstream feeder fuses. The transformer has so far...
  5. BigJohn1

    Temperature Correcting Cable Tests

    I'm looking for any information on temperature correcting insulation resistance and hipot values for low- and medium-voltage cable. I can find general references to correcting things to 15°C but very little in the way of correction tables or published insulation coefficients, so I'm not sure...
  6. BigJohn1

    Switchgear and Xylene?

    Have some folks who would like to use an acetone and xylene based degreaser as an all-purpose switchgear cleaner. Seeing what this stuff can do to plastic makes me believe this is a really bad idea, but I can't find much substantiation against it. I know alcohol is the industry standard and...
  7. BigJohn1

    SF6 Hipot Myth?

    Apparently one of our guys had some commissioning techs from a major circuit-breaker manufacturer tell him that under no circumstances should a high-potential test be performed on their SF6 interrupter bottles. I cannot find any data to substantiate this practice nor provide any technical...
  8. BigJohn1

    Intermittent Generator Differential Fault

    I have a 480V 1MW utility-paralleled generator that is tripping on phase differential. It will go months between events where it operates fine at all levels of load, but then a couple trips will happen in close succession. There doesn't appear to be any common factor between trips events. The...
  9. BigJohn1

    CT Excitation and Knee Point

    I'm getting into CT testing, and am having trouble understanding the value of a knee-point excitation test. The majority of the tests I've done are on 600V CTs between 500-5000A. The knee point values are usually in the mA range at a couple of hundred volts. My understanding is the ANSI knee...
  10. BigJohn1

    Loud Transformer and Harmonic Mitigation

    We've got an unusual problem: 75kVA 480Δ/208Y transformer with a "noise issue". It's within NEMA decibel specs for a transformer that size, but the customer still doesn't like the sound because they work around it. We actually did a load test on a purely resistive load, and it was nice and...
  11. BigJohn1

    Polarization Current VS Absorption Current

    When doing insulation resistance testing, I understand there are two different measurement ratios: Dielectric Absorption Ratio and Polarization Index. I think I have a good understanding of polarization current being measured during the PI test where the dipoles in the conductor insulation are...
  12. BigJohn1

    Generator Instantaneous Overcurrent Trip

    I have a re-occurring problem where a simple 1.5MVA 600V hydroelectric generator periodically goes out on "instantaneous" overcurrent for not obvious reason. The unit has been meggered and there are no faults. It will go online and run fine for days sometimes, and then some days it will trip a...
  13. BigJohn1

    Generator Excitation & Power Factor

    I'm trying to visualize the relationship between the exciter field in a synchronous generator and the generator's power factor. Double check me on this: When synchronizing the generator, increasing the field current will increase the flux density, which increases the voltage induced in the...
  14. BigJohn1

    Huge Arc Flash Boundaries

    The utility I work for recently had an arc flash analysis done for our plants. It was a given that we had equipment that was "Dangerous" and would have large arc flash boundaries. What I wasn't expecting was to find that there are a bunch of boundaries in excess of 100 feet, and sometimes much...
  15. BigJohn1

    Sulfated Battery Photos?

    I wasn't sure where to ask this, but it's related to power generation, so... I am looking for photos of sulfated flooded-cell batteries. Our station operators do a visual check of the battery plants, and I'd like to have pictures to help them recognize a sulfated battery because some of these...
  16. BigJohn1

    Faraday Cage - Principles of Operation

    I'm trying to understand how a faraday cage actually serves to provide electromagnetic shielding. What I've read seems to suggest that a true "Faraday cage" is a mechanism that only applies to electrostatic charges; as best as I understand it the charges applied "cage" are distributed equally...
  17. BigJohn1

    Transformers & Capacitive Coupling

    I've heard a lot mentioned lately concerning capacitive coupling as it relates to power transformers. Needless to say, I don't understand how it comes into play. I was under the impression that transformers were inductively coupled, that the change in current and corrosponding change in...
  18. BigJohn1

    Current Flow

    First note, I'm an electrician and not an engineer, so if these questions are a bit low-brow for this site, I appologize. However, I'm having a diffricult time finding decent explanations for why current behaves the way it does. First things first, I don't understand how neutral current...

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